Add Adabas access method

[neilbeesley]

There is a GVBMR95 read exit already that can be used to access Adabas data during extract processing (GVBXRA).
However, this is a read exit and not an access method, so uses OP/RD/CL calls like other read exits. Also it contains some "customized" information relating to Adabas access which cannot yet be defined through the Workbench.
This suggestion is to provide an access method for Adabas, implemented in a similar way to GVBMRBS/GVBMRSQ and allow the necessary Adabas information to be defined in the Physical File definition through the workbench.

[neilbeesley]

The existing read exit for Adabas works fine but requires the user exit to contain certain Adabas information needed to read the database. For example:

DBID and file number
Returned record length
Adabas search and value buffers (i.e. selection criteria)
Adabas format buffer (i.e. description of fields, formats and lengths returned to the LR).
Optionally (multi-fetch) blocking factor, if not the default 32K / LRECL

It would be more consistent with other types of file access methods to put this logic into a defined SAFR IO module for Adabas, e.g. GVBMRAD and allow the required parameters to be defined through the workbench.

Notes:
The returned record length should match the LR -- provided the Adabas format buffer is correctly defined
The Adabas value buffer may be a mute point as we normally start at the beginning of a file -- but this does allow to start at a particular value with Adabas
The Adabas format buffer is the fields and optionally the formats and lengths used to populate the contents of the returned records.